Synthesis and phase transformation of Al–Cu–Fe coatings by various processes

Introduction

Al–Cu–Fe quasi-crystalline materials (QCs) have attracted a great deal of attention in recent years because of its variety of applications. Even though quasi-crystals are mostly metallic alloys, they exhibit low thermal conductivity, electrical conductivity (typical of semiconductors), remarkable tribological behaviour and good resistance to corrosion.1 ^– 3 It could be considered as mirrors or filters with very good resistance to abrasive wear and because of very low adhesiveness for use in components.

The QCs are also known to be promising candidates for use as functional coating due to their properties, such as high hardness, high Young's modulus, high wear resistance, low friction coefficient, superplasticity at elevated temperatures and thermal expansion coefficients comparable with those of metals.4 ^– 8

However, the mechanism of QC phase in such materials is strongly dependent on the composition and surface energy because of its complex components and low stability. At the same time, the change in QC grain size essentially affects their physical properties. In this case, the control of process is important for the industrial application of QC Al–Cu–Fe materials; currently, there is a lack of information in the literature regarding the systematic investigation.

Here, a sputter deposited process can be developed as a cost effective approach for the synthesis of Al–Cu–Fe coatings. Sputter deposition is a novel technique for the synthesis of advanced materials. 9 9,10 In this technique, the coatings can be prepared on large area substrates and controlled over the composition. The structures and physical properties of coatings were highly influenced by the deposition parameters, such as the annealing temperature and time, the annealing ambience and the sputtering power.

In this article, the synthesis and structural evolution of Al–Cu–Fe QC coatings using different deposition methods and annealing processes were investigated.

Experimental

Coatings of Al–Cu–Fe were prepared on Si (100) substrates using dc magnetron co-sputtering from a homemade circular planar magnetron sputtering system. The sputtering powers of aluminium, copper and iron targets were controlled at various sputtering powers (Al/Cu/Fe) of 113/18/44 W, 113/13/44 W and 113/13/35 W. The deposition chamber's base pressure was 8··0×10⁻⁵ Pa, and during deposition, the gas pressure was 1··0 Pa, and the flowrate of argon gas was set at 5··0×10⁻⁷ m³ s⁻¹. The substrate to target distance was 140 mm. Deposition was performed at a substrate temperature of 25°C for 180 min. In this study, the samples were annealed at different temperatures and times (room temperature, 550°C for 5 h and 750°C for 10 h).

The surface morphology was examined by means of atomic force microscopy (AFM, DI Nano-scope IIIA Multimode). The AFM image analysis was carried out using commercial WSxM 4··0 (Nanotec Electronica) software procedures to determine the surface roughness characterised by the root mean square parameter. In addition, the values of average grain size were measured by number counting method over a series of grains associated with a line in the two-dimensional AFM image in the area of 1000×1000 nm. The X-ray diffraction with Cu target (Rigaku D/MAX2550) was used to determine the structure of the coatings. The chemical composition of the coatings was analysed by scanning electron microscopy (JSM-6360LV) with energy dispersive X-ray spectroscopy (EDAX-Falcon).

Results

Figure 1 shows the X-ray diffraction patterns of coatings (113/18/44 W) as a function of annealing temperatures and times. None of the annealing processes showed broadening of peaks, in which the structure is found to be amorphous. Increasing annealing temperatures and times caused the appearance in structure of λ-Al₁₃Fe₄ phase to 550°C, β-Al₅ (Cu, Fe)₅ and ω-Al₇Cu₂Fe phase to 750°C, as shown in Fig. 1. This indicates that in the annealing process, the energy supplied was sufficient to convert the Fe atoms to its 3+ oxidation state and results in phase formation.11

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Figure 1

X-ray diffraction patterns of Al–Cu–Fe coatings deposited at sputtering power (113/18/44 W)

The variation of the crystal phase of the Al–Cu–Fe coatings as the decrease in Cu power is shown in Fig. 2. None of the annealing processes showed λ-Al₁₃Fe₄ phase grown from the amorphous structure. Increasing annealing temperatures up to 550°C showed a clear phase change from the λ-Al₁₃Fe₄ phase to the ω-Al₇Cu₂Fe and β-Al₅ (Cu,Fe)₅ phases. It is indicated that the decrease in the power of the Cu target can reduce the reaction temperature of the ω and β phases in the coatings. These results show that high crystallinity can be obtained at high sputtering power ratios (Al/Cu and Fe/Cu) due to the surface diffusion of these atoms (Al and Fe) and, consequently, to the addition of surface mobility of the atoms on the substrate surface.12

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Figure 2

X-ray diffraction patterns of Al–Cu–Fe coatings deposited at sputtering power (113/13/44 W)

According to Fig. 3, increasing annealing temperatures up to 550°C showed that θ-Al₂Fe and ω-Al₇Cu₂Fe disappeared completely, which changed into β-Al₅ (Cu,Fe)₅ and i-Al₆₂Cu₂₆Fe₁₂ phases with the decrease in Fe power from 44 to 35 W. In addition, the λ-Al₁₃Fe₄ peaks disappeared with the increase in i-Al₆₂Cu₂₆Fe₁₂ phase at a higher temperature of 750°C, indicating that alloying of Al, Cu and Fe was completed. This result may be due to the fact that the decrease in the power of the Fe target can reduce the reaction temperature of β and i phases in the coatings.

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Figure 3

X-ray diffraction patterns of Al–Cu–Fe coatings deposited at sputtering power (113/13/35 W)

Figure 4 shows the AFM images of Al–Cu–Fe coatings prepared at sputtering powers of 113/18/44, 113/13/44 and 113/13/35 W at room temperature, 550 and 750°C annealed respectively. It can be observed that using a higher temperature annealing process can increase the mean surface disturbance and affect the mean area of the coatings. Obviously, the surface roughness decreases with increased annealing temperatures and times. There is an inverse relationship between the mean surface disturbance of the coatings and the phase transformations of β and i phases.13, 14

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Figure 4

Images (AFM) of Al–Cu–Fe coatings deposited at different powers (113/18/44, 113/13/44 and 113/13/35) for room temperature, 550 and 750°C annealed respectively

At the same time, the change in QC grain size with the decrease in power of Cu and Fe can be found in Fig. 4. It seems that the reduction in Al, Cu and Fe powers may have a significant increase in grain height and width. This means that the control of power processes has a strong effect in changing the composition of the i phase.15 Furthermore, a significant change in the shape and particle size of the QC grain can be observed in Fig. 4, which implies that changing the annealing condition can also control the formation and growth of the QC grain.

Discussion

Phase diagram for Al–Cu–Fe system

The Al–Cu–Fe ternary phase diagram is concluded by energy dispersive X-ray spectroscopy results with various powers (Al, Cu and Fe) annealed at 750°C in Fig. 5. According to the ternary phase diagram for the Al–Cu–Fe system, it is expected that a chemical composition gradient is formed by the application of phase transformation of i-Al₆₂Cu₂₆Fe₁₂ phase in the deposition. The local chemical composition around the ring's outer region changes along the arrow, as seen in Fig. 5, since the atomic ratio (Cu/Fe) of i-Al₆₂Cu₂₆Fe₁₂ is larger than that of Al₃CuFe₉. In addition, θ-Al₂Fe, β-AlFe, β-Al₅ (Cu,Fe)₅, ω-Al₇Cu₂Fe and λ-Al₁₃Fe₄ phases occurred together, which indicated that these phases come from the solid state reactions reported by Wang et al.16

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Figure 5

Al–Cu–Fe ternary phase diagram with various powers at 750°C

Conclusion

Based on the results from the above experiments, the following conclusions can be drawn.